Re: [PATCH v4 5/9] pcie_sriov: Validate NumVFs

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Feb 14, 2024 at 11:49:52PM +0900, Akihiko Odaki wrote:
> On 2024/02/14 15:52, Michael S. Tsirkin wrote:
> > On Wed, Feb 14, 2024 at 02:13:43PM +0900, Akihiko Odaki wrote:
> > > The guest may write NumVFs greater than TotalVFs and that can lead
> > > to buffer overflow in VF implementations.
> > > 
> > > Fixes: 7c0fa8dff811 ("pcie: Add support for Single Root I/O Virtualization (SR/IOV)")
> > > Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> > > ---
> > >   hw/pci/pcie_sriov.c | 3 +++
> > >   1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/hw/pci/pcie_sriov.c b/hw/pci/pcie_sriov.c
> > > index a1fe65f5d801..da209b7f47fd 100644
> > > --- a/hw/pci/pcie_sriov.c
> > > +++ b/hw/pci/pcie_sriov.c
> > > @@ -176,6 +176,9 @@ static void register_vfs(PCIDevice *dev)
> > >       assert(sriov_cap > 0);
> > >       num_vfs = pci_get_word(dev->config + sriov_cap + PCI_SRIOV_NUM_VF);
> > > +    if (num_vfs > pci_get_word(dev->config + sriov_cap + PCI_SRIOV_TOTAL_VF)) {
> > > +        return;
> > > +    }
> > 
> > 
> > yes but with your change PCI_SRIOV_NUM_VF no longer reflects the
> > number of registered VFs and that might lead to uninitialized
> > data read which is not better :(.
> > 
> > How about just forcing the PCI_SRIOV_NUM_VF register to be
> > below PCI_SRIOV_TOTAL_VF at all times?
> 
> PCI_SRIOV_NUM_VF is already divergent from the number of registered VFs. It
> may have a number greater than the current registered VFs before setting VF
> Enable.
> 
> The guest may also change PCI_SRIOV_NUM_VF while VF Enable is set; the
> behavior is undefined in such a case but we still accept such a write. A
> value written in such a case won't be reflected to the actual number of
> enabled VFs.

OK then let's add a comment near num_vfs explaining all this and saying
only to use this value. I also would prefer to have this if
just where we set num_vfs. And maybe after all do set num_vfs to
PCI_SRIOV_TOTAL_VF? Less of a chance of breaking something I feel...

-- 
MST